1. Field of the Invention
The present invention relates to a head substrate, printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to a head substrate prepared by forming, on the same substrate, electrothermal transducers for generating heat energy necessary to print, and a driving circuit for driving the electrothermal transducers, a printhead using the head substrate, a head cartridge using the printhead, and a printing apparatus.
2. Description of the Related Art
The electrothermal transducer (heater) of a conventional inkjet printhead (to be referred to as a printhead hereinafter) and a driving circuit for the electrothermal transducer are formed on the same substrate by a semiconductor process technique as disclosed in, for example, U.S. Pat. No. 6,290,334.
Recent printheads are achieving high print speeds and high image qualities, and the number of arrayed segments is increasing. Since many segments are driven at high speed, power consumption increases, and as a result, the temperature of the printhead rises. The temperature rise of the printhead leads to an ink discharge failure and fluctuations in the amount of ink discharge, degrading the print image quality.
Of building elements of the printhead, a converted voltage generation circuit consumes large power, in addition to a heater which heats ink. At least one converted voltage generation circuit is arranged on a substrate common to a driving circuit. When a plurality of circuits are arranged on one head substrate in correspondence with a plurality of inks in order to discharge these inks for color printing, a plurality of converted voltage generation circuits are often arranged on the same substrate. As a result of increasing the number of converted voltage generation circuits, power consumption increases.
FIG. 11 is a circuit diagram showing an example of a conventional converted voltage generation circuit and its peripheral circuit.
Part of FIG. 11 except for a converted voltage generation circuit 300 shows an equivalent circuit for one segment. A converted voltage VHTM output from the converted voltage generation circuit 300 is commonly used by level converters 307 in a plurality of segments. The level converter 307 boosts, to a signal of the converted voltage VHTM, a signal of a logic power supply voltage (e.g., 3.3 V) for operating a logic circuit such as a shift register. The output voltage from the level converter 307 is applied to the gate of a MOSFET serving as a switching element (driving element) 305. The switching element 305 is series-connected to a heater 304. The converted voltage generation circuit 300 uses, as a power supply, the same voltage VHT as a heater voltage VH of about 24 V applied to a heater. The converted voltage generation circuit 300 is formed from a resistance element including a diffusion resistance or polysilicon element, and a MOSFET 306.
The converted voltage generation circuit takes the form of a source follower circuit. By applying a predetermined reference voltage to the gate of the MOSFET 306, the value of the converted voltage VHTM is determined. Since a constant voltage is always applied to the gate of the MOSFET 306, this circuit arrangement can suppress variations of a converted potential even if a current abruptly flows through the drain-source path of the MOSFET 306. To always keep the converted potential constant, a constant voltage must always be applied to the gate of the MOSFET 306.
As an example of a reference voltage generation portion 303, a dividing resistance contributes to generating a predetermined reference voltage in FIG. 11. The resistance element is desirably an element (e.g., polysilicon element) whose resistance value hardly varies by heat.
However, this circuit arrangement consumes a large amount of power because a through current always flows through the reference voltage generation portion.
In addition, the resistance element used as the dividing resistance poses a problem. A resistance element used for a semiconductor is generally a diffusion resistance whose layout area is small. However, the diffusion resistance changes depending on the bias voltage and is not an ideal element used as the dividing resistance. For this reason, the above-described conventional art adopts a metal resistor or polysilicon resistor independent of the bias voltage. However, a resistor of this type requires a large layout area on the head substrate, increasing the chip size and raising the manufacturing cost of the head substrate.